Energy storage is important in many energy consumption applications, including conventional and renewable utility power generation, building air-conditioning and heating, and industry process heating. For example, nuclear and coal-fired power plant need to shift their generated power during low demand hours to peak demand hours of the grid; wind power and solar power plants due to their intrinsically intermittent nature, need even more energy storage solution since their energy resources are more uncontrollable to meet the grid demand, which may introduce instability for the grid.
Direct electric energy storage requires batteries. Although there are continued efforts to improve battery technologies, their intrinsic high cost nature limits their applications to small scale emergency power supplies. Hydroelectric and compressed air solutions are two current applicable solutions, where electricity is first converted to potential energy of water or air by pumps and compressors, and then converted back to electricity when needed. However, these two solutions all require special geologic conditions, i.e. geologic conditions to build two low and high attitude water reservoirs or natural underground air-tight high pressure air-reservoir, which are rarely available in local power plants.
Thermal energy storage, however, is intrinsically low cost, due to availability of various low cost materials for the application. Since most of power plants (more than 80%) generate electricity through thermal process, thermal energy storage can be conveniently applied to utility power generation.
In the field of electricity generation, most power plants use Steam Turbines as driving engines for electricity generation. Steam Turbines work on the principle of the Rankine cycle, such as via turbine blades that are driven by expansion of overheated steam. The efficiency is determined by the pressure ratio of steam at entry and outlet. To increase the efficiency, the steam at the outlet needs to be condensed, most often by cooling water. In this process, large amount of latent heat, usually more than 60% of total thermal energy at entry is released from the low pressure steam at the outlet, e.g., dumped into cooling water and dissipated to the environment. For one ton of steam, this cooling process requires 50 to 100 tons of fresh water, which consume 5 to 10% of the electricity generated by turbine engine for the pumps to perform water circulation and 5 to 10 tons of water loss in water tower cooling process. If the water source is not free, the cost of electricity and water will consist 20-30% of total operating cost. As a result, power plants cannot be built at any location even if other environmental effects are not considered. Similar cooling process is also required for large scale refrigeration and desalination process. In these processes, water cooling operation cost also composed of large percentage of the total operating cost.
These are great needs to eliminate water cooling while maintaining the efficiency of electricity generation, refrigeration, and desalination from both environmental and economic grounds. In the past, air-cooling methods have been tried, however, not very successfully due to expensive equipment and the high electricity consumption involved.
Thermal energy storage is also important to concentrated solar power (CSP) plants. The working principle of CSP is to use various forms of focusing mirrors, such as parabolic dish mirrors, parabolic trough mirrors, Fresnel mirrors, and other types of focusing mirrors to focus the solar light onto the thermal collector, where special coating converts the solar radiation into thermal energy; thermal power heats up the heat transfer fluid (HTF) which flows through the thermal collector to a certain temperature, then generates high pressure high temperature steams via heat exchanger to drive the steam turbine for the electricity generation. In other words, CSP uses a solar boiler to replace a regular fossil fuel or nuclear fuel boilers, keeping other portions of conventional power plant unchanged.
However, the unstable nature of solar radiation, caused either by clouds or sunset, requires a thermal storage sub-system in CSP plants in order to qualify them as a base load power supplier. Therefore, a low cost and highly efficient thermal storage solution becomes a key for CSP plants to be deployed in large scale to replace fossil fuel power plants. For example, for a given parabolic trough CSP power plant, without thermal storage sub-system, its annual operation coefficient will be about 20%, i.e., 1760 operation hours per year; if a thermal storage sub-system is used, the operating coefficient can be increased to more than 60% or 5260 operation hours per year.
For thermal storage applications, there are three key thermal media: heat transfer fluid (HTF), thermal storage medium, and working medium. HTF transfers the heat from heat generator or collector to thermally charge storage medium or directly heat working medium through exchanger; storage media receive the heat from HTF and deliver the heat to the working medium through heat exchanger; and the working medium receives the heat from heat exchanger and drives the heat engine.
HTF can either be a gas or liquid. There are mainly two types of liquid HTF: one is heat conducting oil and the other molten salt. Normally, the highest temperature conducting oil can sustain is about 400° C. Above this temperature, the conducting oil will decompose. Molten salt, on the other hand, can sustain up to 600° C. temperature, however, has to be kept at higher than 220° C. all the time in order to avoid solidification and subsequent damage to the transport pipes and containers. This requirement normally causes higher system maintenance costs. For gas type HTF, steam can be used; however, its pressure and cost will be too high for high temperature steam applications; hot air can be used as gas type HTF; however, it has very low heat capacity and needs high electricity consumption to create high flow rate.
In most cases, the working medium is a liquid, such as water, which is pre-pressurized to a desired working pressure, then heated to the desired working temperature via a heat exchanger to undergo a liquid-gas phase transition. High pressure vapor at the heat engine entry will expand, and during the process, the potential energy of pressurized molecule transform to kinetic energy due to the high speed gained during the expansion. This kinetic energy is the driving force of the heat engine for the mechanical work and, finally, electricity generated. For example, for a typical 1 MW steam turbine, it requires 2.4 MPa pressure at 355° C. temperature to achieve highest efficiency. For a typical 100 MW steam turbine, the required steam pressure and temperature will be 10 to 12 MPa at 380 to 400° C. temperature range. Large size steam turbines can usually achieve higher conversion efficiency from thermal energy to electricity, but in order to achieve this, they also require steam of higher pressure and temperature.
Two techniques to store thermal energy based on the types of heat absorbed in materials include methods of sensible heat and methods of latent heat.
Sensible heat storage mechanisms may be based on the specific heat capacity of the storage medium, wherein the charging and discharging of thermal energy to and from the storage medium may be realized by increasing or decreasing the temperature of the materials: Q=MCp(T2−T1)=MCpΔT (Eq. 1), Where Q is the sensible heat stored in, M the mass, Cp specific heat capacity of the storage medium, T1 and T2 starting and ending temperatures, ΔT the temperature difference.
Sensible heat storage is the most common, simple, mature and widely used thermal storage method. It can be further classified into four different methods: liquid, solid, liquid and solid mixed, and pressurized vapor sensible heat storage.
Liquid-phase sensible heat thermal storage. Liquid phase sensible heat thermal storage devices normally use either direct or indirect heat exchange methods. Here, for example, CSP solar collecting field, such as parabolic trough or linear Fresnel mirrors, normally use conducting oil (mineral oil or synthetic oil) as its HTF while using molten salt as liquid phase sensible heat thermal storage material. Liquid-phase sensible thermal storage materials are most often used in so called “Active Thermal Energy Storage” system, where storage materials circulate through heat exchangers and collectors. In this way, one needs to use a heat exchanger to transfer thermal energy from conducting oil to molten salt to store the thermal energy. Therefore, this method is called indirect thermal storage. Currently, most distributed solar thermal collecting fields (such as parabolic dish, parabolic trough, and linear Fresnel CSP) use such methods, which are the only commercialized mature thermal energy storage method. Typically, two tanks are used, one for the high temperature molten salt and the other one for the low temperature molten salt. During thermal energy storage time, the high temperature conducting oil will heat up the low temperature molten salt when it flows from low temperature container to high temperature container via a heat exchanger to transfer thermal energy from high temperature conducting oil to molten salt, while the high temperature molten salt will be stored in the high temperature container. When solar energy is not available, the high temperature molten salt will flow into low temperature container via a heat exchanger to generate high temperature high pressure steam for continuous electricity generation. This process comes to an end when most of the high temperature molten salt flow out from the high temperature container.
There are several problems with this approach: a) it requires several high temperature specialty pumps that can handle high temperature and very corrosive molten salt between the two containers, conducting oil-molten salt heat exchanger and molten salt-steam generation heat exchanger; b) it requires specialty heat exchanger due to the natures of molten salt; c) the construction cost is still quite high: for example, for large scale deployment, the thermal storage device construction cost can be $40/kWh of heat.
The two container molten salt solution can also become a direct thermal energy storage sub-system for a parabolic trough or tower CSP system. In these cases, the molten salt acts both as HTF for the solar collecting field and liquid phase sensible heat thermal storage material, i.e., HTF and sensible heat thermal storage material become the same material, no extra heat exchanger is involved, therefore, called direct thermal energy storage. Obviously, this approach avoids a heat exchanger, which will reduce thermal energy loss during the process. It is suitable for parabolic trough system works at 400˜500° temperature range. A main shortcoming with liquid-phase molten salt sensible heat thermal storage approach is that extra heating devices and energy are required to keep the molten salt temperature above 220°, which is common molten salt's melting point, in order to avoid damage to the transport piping system during its solidifying process. For a distributed solar collecting field, this will significantly increase the complexity and the cost for the transport pipe, both in their construction and in their maintenance and services.
Tower CSP system can normally use direct liquid phase sensible heat thermal energy storage solution, such as in Spain Solar Tres tower CSP power plant. Because the transport piping system is vertically installed in the CSP tower, the liquid molten, salt is easily discharged from the pipes so that the solidifying problem is not as severe as in the parabolic trough CSP system. In addition, since the working temperature of tower CSP is normally significantly higher than that of a parabolic trough CSP system, the sensible heat thermal storage approach is more suitable to the tower CSP than for the trough CSP. For proper liquid phase temperature range, normally a mixture of inorganic salts or a single phase compound is used in such application. For example, the Solar Two tower CSP at Nevada of US used 60% of sodium nitride and 40% of potassium nitride as a single phase compound; its melting point is 220°. Its working temperature range is 300˜600°. The SEGS trough system that built at California desert of USA in the 1990s used therminol VP-1, Hitech (53% KNO3+7% NaNO3+40% NaNO2 mixture) and Hitec XL (45% KNO3+48% Ca(NO3)2+7% NaNO3 mixture) as their direct liquid sensible heat thermal energy storage materials.
Solid state sensible heat thermal energy storage. Solid state sensible heat thermal energy storage uses rock, concrete, sand, etc. low cost solid state material as thermal storage media. Since the solid materials cannot be transported between containers for thermal energy transportation, a gas phase or liquid phase HTF also have to be used for heat exchange media between HTF, storage medium and working medium. This type of system also called “Passive Thermal Energy Storage” system. In direct steam generation CSP system, the thermal storage system normally uses solid state sensible heat thermal energy storage materials. The greatest advantage is low cost for storage materials. However, it can only be used in indirect thermal energy storage approach. Tamme from Germany Aero Space Center (DLR) studied and developed high temperature concrete and casting ceramic as solid state sensible heat thermal energy storage material based on the property study of sand-rock concrete and basalt concrete, where the frame for the high temperature concrete is ferric oxide, the cement acts as filling material. However, a disadvantage of solid state sensible storage method is that the heat exchange and working temperature decrease during discharge, since sensible heat thermal storage materials' temperature decreases as thermal energy decreases. Another problem is that the thermal conductivity and heat transfer coefficients are low. Also if direct generated steam is used for the HTF, as it currently is, this requires the transport piping system to cross the entire solar collecting field and the thermal storage containers to sustain high temperature and high pressure. This will dramatically increase the cost for such steam transport pipes as well as the thermal storage container cost. On the other hand, to reduce the cost, the pressure of the directly generated steam has to be lowered, which will decrease the working efficiency for steam turbine. As consequence, this approach has been researched for a long time without necessary breakthroughs.
Liquid-Solid Combined Sensible Heat Thermal Energy Storage.
Liquid-solid state combined sensible heat thermal energy storage approaches use some solid state materials and HTF that is compatible at high temperature so that the solid state material and the HTF can be combined together to increase the heat capacity for the combined thermal storage system. One of the obvious advantages of using solid state material in thermal storage is to significantly reduce the usage of HTF while keeping the total amount of thermal energy stored unchanged so that the thermal storage cost can be lowed (in general; the cost of solid state material is much lower than that of HTF. In order to reduce the equipment investment cost for the two tank liquid phase molten salt thermal energy storage system, James from Sandia National Libratory designed and tested a thermocline tank storage system with 2.3 MWh thermal storage capacity. The thermocline tank storage system utilizes thermocline layer formed due to natural temperature cline distribution based on the relationship between thermal storage material density and the temperature. This thermocline layer is formed when there is a temperature difference appears between the top (high temperature portion) and the bottom (the low temperature portion). This thermocline layer acts as an insulation layer so that the molten salt on its top can keep at higher temperature and the molten salt on its bottom can keep at lower temperature. During thermal energy storage period, the thermocline layer moves to upper direction. During thermal energy release period, the thermocline layer moves to lower direction. In this way, it can keep the output molten salt at a constant temperature. When the thermocline layer reaches the top of the tank or to the bottom of the tank, the temperature of the output molten salt will change dramatically. In order to maintain the temperature layer gradient, one needs to strictly control the amount of input and output molten salt, as well as properly arranges solid state filling material into layered structure, paired with floating inlet and ring-shell heat exchanger devices. Although this approach may reduce the thermal storage cost by 35% comparing with previously described liquid phase sensible heat thermal energy storage system, it still has the similar shortcomings mentioned before.
Pressurized Water (Steam) Thermal Energy Storage.
The CSP power plant of Planta Solar 10 at Seville of Spain uses pressurized steam at 285° C. with 4 MPa pressure to store the thermal energy. PS10 is the first tower CSP project in Spain. It needs high pressure container to store the pressured high temperature water directly flowed from heat source or collector through high pressure pipes. This thermal storage approach can only be used to smooth the solar radiation intensity fluctuations during the day, which can provide 1 hour of steam to the turbine generator. When the pressured high temperature water is released from the storage vassal, it undergoes liquid-gas phase transition as the pressure is slightly reduced. The high pressure steam can be used directly to drive steam turbine. Strictly speaking, the stored energy here is still sensible heat from high pressure water, not latent heat which only exists upon liquid-gas phase transition outside the storage tank. It is an effective method to provide balance load for steam turbine. However, due to high cost of pressurized vessels, this approach is very hard to be deployed in large scale.
Latent heat storage mechanism utilizes the heat associate with a materials' physical state change, such as liquid to gas, solid to liquid, solid crystalline phase to phase transition. Latent heat associated with the transition has much higher effective specific heat capacity within transition than that of sensible heat storage materials. Heat absorbed or released at the transition is described by: Q=MCp(eff)δT (Eq. 2), where M is the mass of the materials and Cp(eff) the effective heat capacity within phase transition, δT temperature difference within transition range.
However, liquid-gas phase change latent heat storage is difficult to use due to extremely large volume change at the phase change. Water, for instance, expands 1600 times when it vaporizes at 0.1 MPa pressure (one atmosphere). Therefore, it is not economical to utilize latent heat for thermal energy storage with liquid-gas phase transition, because a large container with very high pressure inside the storage container is required to accommodate the gas phase volume, resulting significantly reduced thermal energy storage density and difficult mechanical structure design for the thermal storage apparatus.
In Europe, 13 countries proposed a design of PCM storage system, referred to as the DISTOR project. In this project, direct generated steam or (high pressure water) is used as HTF, where graphite and PCM micro-encapsulated compound storage materials are used. Other methods with mixed PCMs have also been proposed. In such previous proposals, heat exchange between HTF/WF and storage materials are shell-piping heat exchanger, where HTF/WF flow in piping and PCMs surround piping inside the tank with solid filling materials to improve the thermal contact.
Although there are many studies on PCM thermal storage, there are still major difficulties of using solid-liquid phase change materials (PCM) as latent heat storage. The first is that PCM volume changes during phase transition. The volume change makes mechanical system design considerably difficult. The second is difficult to maintain good heat conduction between solid-liquid PCMs and HTF. Heat transfer between the storage medium, HTS and working medium has not been solved properly, as a result, no commercial application of latent heat storage method and apparatus have succeeded so far.
There is, therefore, a need to overcome the shortcomings of the current thermal energy transfer/storage method.